In the drilling of oil and gas wells, drilling fluid such as, for example, a variety of drilling muds known in the art, is conventionally pumped through the drill string via a connection at the top of the drill string in order to circulate the drilling fluid through the drill string during drilling operations. The drilling fluid is typically held in drilling fluid tanks or reservoirs and pumped from a stand pipe to a Kelly drive or top drive, which in turn is connected to the top of the drill string. As drilling progresses during the drilling process, portions of drill pipe are added (e.g., as 90 ft stands, 30 ft individual pipe sections, and/or a variety of other drill string sizes known in the art) between the top drive and the drill string in order to extend the drill string into the formation. Each time a drill string connection (also known as a “hook”) must be made, the circulation of drilling fluid must be stopped. Conventionally, drill string connections are made by shutting down the pumps used to circulate the drilling mud before making the drill string connection, disconnecting the Kelly drive or top drive from the drill string, and connecting a stand or pipe section to the drill string. With the drill string connection made, the Kelly drive or top drive may be reconnected to the new stand or pipe section and the pumps restarted to again circulate the drilling fluid through the drill string. With the drilling fluid again circulating through the drill string, drilling operation may then continue.
One drawback to the method of the prior art is the loss of equivalent circulating density (ECD) during a hook. ECD is the effective density exerted by a circulating fluid against the formation, taking into account the mud density and the pressure drop in the annulus at a given point in the annulus. In other words, when mud is being circulated, because of friction in the annulus as the mud is pumped, there is an increase in bottomhole pressure. This pressure is significantly higher than when the mud is not being pumped. The ECD is an important parameter for drilling operations because loss or a drop in ECD can result in kicks and losses in the annulus.
The need to stop drilling fluid circulation in the drill string in order to make drill string connections is time consuming and disruptive to drilling operations. Therefore, what is needed is an improved system and method for circulating drilling fluid.